Heretofore, various methods have been proposed to obtain inorganic spheres having a uniform particle size. U.S. Pat. No. 5,278,106 discloses a method for producing inorganic spheres which comprises injecting an aqueous liquid into an organic solvent through a microporous membrane to prepare a W/O type emulsion and converting droplets in the resulting W/O type emulsion into inorganic spheres.
The particle size distribution of the emulsion can be narrowed by this method, but this method is insufficient in terms of uniformity in the particle size of the inorganic spheres, since the flow of the organic solvent is not controlled, which causes an emulsion particle size distribution. Further, because a glass microporous membrane with poor alkali resistance has problems with its durability such as erosion of the pores to larger size by an aqueous solution containing an alkali metal silicate as the aqueous liquid, a W/O type emulsion having a uniform particle size can not be obtained continuously and stably.
In recent years, U.S. Pat. No. 6,576,023 proposed a method and an apparatus for producing a homogenous emulsion by injecting a pressurized aqueous solution containing an inorganic compound through distorted micropores into an organic liquid. Recently, development of a method and an apparatus which allow long-term efficient and stable mass production of inorganic spheres having a uniform particle size has been desired.